Add jest for unit testing logic

There are a number of non-test files in src that have logic and this will likely
continue to grow.

.eslintignore

@@ -1,2 +1,3 @@
 report
 package-lock.json
+coverage

.eslintrc.json

@@ -2,7 +2,12 @@
 	"extends": [
 		"wikimedia",
 		"wikimedia/node",
-		"wikimedia/language/es2021"
+		"wikimedia/language/es2021",
+		"plugin:jest/recommended",
+		"plugin:jest/style"
+	],
+	"plugins": [
+		"jest"
 	],
 	"env": {
 		"browser": true,
@@ -20,6 +25,9 @@
 			}
 		]
 	},
+	"parserOptions": {
+		"ecmaVersion": 13
+	},
 	"settings": {
 		"jsdoc": {
 			"mode": "typescript"

jest.config.js

@@ -0,0 +1,28 @@
+/*
+ * For a detailed explanation regarding each configuration property, visit:
+ * https://jestjs.io/docs/configuration
+ */
+
+module.exports = {
+	// Automatically clear mock calls, instances and results before every test
+	clearMocks: true,
+
+	// Automatically restore mock state and implementation before every test.
+	restoreMocks: true,
+
+	// Indicates whether the coverage information should be collected while executing the test
+	collectCoverage: true,
+
+	// An array of glob patterns indicating a set of files for which coverage
+	// information should be collected
+	collectCoverageFrom: [
+		'src/*.js'
+	],
+
+	coverageDirectory: 'coverage',
+
+	// An array of regexp pattern strings used to skip coverage collection
+	coveragePathIgnorePatterns: [
+		'/node_modules/'
+	]
+};